1. Technical Field
The present invention relates to a wire feed apparatus for automatically extending wire electrodes (hereinafter referred to as wires) in a wire cut electric discharge machine, and more particularly, to a wire feed apparatus in which a wire conveyor belt is shunted from an automatic wire extension position at the time of electric discharge machining so that wear of the belt and variation in wire feed speed can be prevented.
2. Background Art
In general, a wire cut electric discharge machine is provided with an automatic wire extension apparatus for automatically stretching a wire which is artificially cut or snapped during machining. At the time of automatic wire extension, the wire is driven toward a workpiece by means of a send roller disposed in the vicinity of an upper guide located above the workpiece, and passes through an initial hole formed in the workpiece or a through hole formed in the middle of a machining path for the workpiece, and a lower guide disposed below the workpiece. Further, the wire is held between a pair of conveyor belts of a wire feed apparatus disposed on the lower-course side of the lower guide, and is transported toward a feed roller by means of these belts. After the automatic wire extension is finished in this manner, electric discharge machining is performed while the wire is being transported from the upper-course side of a wire transfer path toward the lower-course side thereof by means of the feed roller.
In the wire cut electric discharge machine with the automatic wire extension apparatus of the aforementioned type, the wire is transported by means of the feed roller at the time of the electric discharge machining after the end of the automatic wire extension, so that the wire feeding function of the wire feed apparatus is available only during the automatic wire extension. Conventionally, however, the wire is held between the conveyor belts of the wire feed apparatus, so that the conveyor belts are circulated so as not to hinder the wire feeding action of the feed roller.
However, the conveyor belts of the wire feed apparatus are not even in total thickness, and extend and contract as the ambient temperature varies. Also, each conveyor belt is constructed so that two opposite ends of a belt element constituting the same are connected in an endless manner, and a stepped portion is formed at the junction. Therefore, the conveyor belts are liable to rotate unevenly. Thus, even though the feed roller is rotated accurately at a predetermined speed, the wire is held between the unevenly rotating conveyor belts, so that the wire feed speed is liable to vary, thereby possibly entailing variation in electric discharge machining characteristic. Moreover, since the wire, transported by means of the feed roller, and the conveyor belts are in contact with one another, the belts can be easily worn away.